The present invention relates to an optical fiber which is suitable for wavelength multiplexed optical transmissions in, for example, a wavelength band of a 1.55 xcexcm wavelength and a wavelength multiplexed optical transmission line using the same optical fiber.
As a transmission network for optical transmissions, a single-mode optical fiber having zero dispersion in a wavelength band of a 1.3 xcexcm wavelength has been laid across the world. Recently, in accordance with development of the information society, information transmission capacities tend to significantly increase. In accordance with such an increase in information, wavelength multiplexed transmissions (WDM transmissions) are widely accepted in the transmission field, and now the age of wavelength multiplexed transmissions has been entered. Wavelength multiplexed transmissions are the method for transmitting a plurality of light signals which are not on one wavelength but are divided into a plurality of wavelengths, which is suitable for large-capacity and high-rate transmissions.
However, in the case where an existing single-mode optical fiber in operation for transmissions which has zero dispersion near 1.3 xcexcm is used and a wavelength band near 1.3 xcexcm is used to carry out wavelength multiplexed optical transmissions, this wavelength band does not coincide with the 1.55 xcexcm wavelength band (for example, 1530 nm to 1570 nm) of the gain bandwidth (including 1500 nm to 1650 nm) of a general optical amplifier using an erbium doped optical fiber. Therefore, in the case where optical transmissions are carried out by using a wavelength band near 1.3 xcexcm, the optical amplifier cannot be used, and therefore, trouble may occur in long-distance optical transmissions. In order to solve this problem, recently, wavelength multiplexed optical transmissions in a wavelength band of 1.55 xcexcm are carried out by using an optical amplifier and an existing single-mode optical fiber having zero dispersion in a wavelength band of 1.3 xcexcm.
However, when optical transmissions are carried out in a wavelength band of 1.55 xcexcm by using the single-mode optical fiber having zero dispersion near 1.3 xcexcm, the existing single-mode optical fiber has positive dispersion of approximately 17 ps/nm/km in a central wavelength of 1.55 xcexcm of the wavelength band of 1.55 xcexcm, and furthermore, the single-mode optical fiber has a positive dispersion slope of approximately 0.06 ps/nm2/km in a wavelength band of 1.55 xcexcm. Therefore, distortion in waveform of the light signals of the respective multiplexed wavelengths increases as the light signals are transmitted in the single-mode optical fiber, separation and distinction of the signals at the receiver side become difficult, the quality of optical transmissions deteriorates, and the reliability of optical transmissions is lost.
Furthermore, as a transmission network for optical transmissions, a dispersion shifted optical fiber whose wavelength of zero dispersion is shifted to be close to 1.55 xcexcm which is the gain bandwidth of an optical amplifier has been proposed. When dispersion in wavelength in optical transmissions becomes close to zero, since a non-linear phenomenon called four wave mixing becomes easy to generate, in particular, in wavelength multiplexed transmissions, a dispersion shifted optical fiber having minute dispersion of a degree at which a non-linear phenomenon is not generated in the wavelengths for optical transmissions has been demanded.
However, if a dispersion shifted optical fiber having the abovementioned minute dispersion is used for long-distance optical transmissions, since the influence of the minute dispersion cannot be ignored, it is difficult to dependently use the dispersion shifted optical fiber having minute dispersion for long-distance large-capacity and high-rate transmissions.
Therefore, in order to solve such a problem, a method has been proposed in which, to compensate for dispersion in the wavelength band of 1.55 xcexcm of a 1.3 xcexcm zero dispersion single-mode optical fiber, an optical fiber having great negative dispersion in the wavelength band of 1.55 xcexcm is inserted into the single-mode optical fiber transmission line, whereby positive dispersion in the wavelength band of 1.55 xcexcm of the single-mode optical fiber is compensated, and deterioration in transmission signals due to chromatic dispersion is suppressed.
As an example of the optical fiber for compensating the dispersion, for example, an optical fiber having a refractive index profile of a single-peak form as shown in FIG. 6 has been proposed. The optical fiber having a refractive index profile of a single-peak form is formed by covering the circumference of center core 1 with a refractive index greater than that of the silica level with outer cladding 5. The optical fiber of the proposed example is formed so that the refractive index of the outer cladding 5 is smaller than that of the silica glass.
However, the dispersion value of the optical fiber having a refractive index profile of a single-peak form in the wavelength band of 1.55 xcexcm is approximately xe2x88x9280 ps/nm/km at most as a limit value in practical use, and therefore, an optical fiber having a smaller dispersion value (absolute value of negative dispersion is great) cannot be realized by means of a refractive index profile of a single-peak form. Therefore, in order to compensate for the positive dispersion of the single-mode optical fiber by an optical fiber with a refractive index profile of a single-peak form, the length required for the optical fiber for dispersion compensation increases, so that it is difficult to reduce the size of an optical fiber for dispersion compensation in which the abovementioned optical fiber is coiled and housed.
Furthermore, in the optical fiber with a refractive index profile of a single-peak form, the dispersion slope in the wavelength band of 1.55 xcexcm is positive, so that it is difficult to compensate for the chromatic dispersion of the single mode optical fiber over a broadband of a 1.55 xcexcm wavelength band.
Therefore, an optical fiber having a W-formed refractive index profile as shown in FIG. 7 has been proposed. The optical fiber having a W-formed refractive index profile is formed so that the circumference of center core 1 with a refractive index greater than that of the cladding level is covered by side core 12 having a refractive index smaller than that of the cladding level, and normally, the circumference of the side core 12 is covered by outer cladding 5 having a refractive index which is almost equal to that of the silica level.
In the optical fiber having the W-formed refractive index profile, the dispersion value in the wavelength band of 1.55 xcexcm can be smaller (absolute value of negative dispersion can be made greater) than that of the optical fiber having a refractive index profile of a single-peak form, whereby an optical fiber whose dispersion value at the wavelength of 1.55 xcexcm is approximately xe2x88x92120 ps/nm/km has become practicable. Furthermore, in the optical fiber having the W-formed refractive index profile, the dispersion slope in the wavelength band of 1.55 xcexcm can be made negative, whereby the positive dispersion slope of the single-mode optical fiber can be compensated to a degree for practical use, so that dispersion over a broadband of a 1.55 xcexcm wavelength can be compensated more than in the case of the optical fiber having a refractive index profile of a single-peak form.
Moreover, for example, in Japanese Laid-Open Patent Publication No. 313750 of 1996, a method is proposed in which an optical fiber having a W-formed refractive index profile whose detailed structure is properly determined is used to compensate for the chromatic dispersion and dispersion slope in the wavelength band of 1.55 xcexcm of the single-mode optical fiber, whereby the chromatic dispersion and dispersion slope in the wavelength band of 1.55 xcexcm are compensated to be almost zero. In addition, a report was presented in Electro-society Convention C-172 1996 by the Electronic Information Transmission Society, stating that chromatic dispersion in a wavelength band of 1500 to 1600 nm was suppressed to be xe2x88x921 to 0ps/nm/km by compensating the dispersion of the single-mode optical fiber by using an optical fiber having a W-formed refractive index profile.
However, the optical fiber having the W-formed refractive index profile is difficult to form so that a negative dispersion slope is provided which can completely compensate for the positive dispersion slope of an optical fiber to be compensated at the wavelength of 1.55 xcexcm being the central wavelength of the wavelength band of 1.55 xcexcm, and dispersion is less than xe2x88x92120 ps/nm/km.
Therefore, also in the case of using the optical fiber having the W-formed profile, the length of optical fiber required for compensation of dispersion of the single-mode optical fiber increases, so that it is difficult to reduce the size of an optical fiber module which is formed by coiling and housing said optical fiber in a case.
Furthermore, in both cases of the single-peak form and W-form of the prior-art optical fibers, the range of light transmission (effective core sectional area) in the single-mode is small, the power density of the light transmitted inside the optical fiber is high, and in addition, as mentioned above, the length of the optical fiber used is long, so that the non-linear phenomenon is easily generated inside the optical fiber. If so, distortion in signal waveform occurs due to this non-linear phenomenon, whereby transmissions cannot be correctly carried out, and therefore, the use of the dispersion compensating device using the optical fibers is inevitably limited.
Moreover, it also can be considered that deterioration of transmission signals due to chromatic dispersion is prevented by using the optical fiber for compensating dispersion of a dispersion shifted optical fiber having minute dispersion, however, it is considered difficult to completely compensate for the dispersion and dispersion slope of a dispersion shifted optical fiber having minute dispersion by the optical fiber having a single-peak or W-formed refractive index profile.
The invention is made in order to solve the abovementioned problems, and a first object thereof is to provide an optical fiber which is short and can compensate for positive dispersion of a single-mode optical fiber having a zero dispersion wavelength at the side of a wavelength shorter than the wavelength band in use at a broadband of the wavelength band in use, and an optical fiber whose effective core sectional area is large, and which can reduce distortion in signal waveforms due to the non-linear phenomenon. A second object is to provide an optical transmission line, in which distortion in signal waveforms due to dispersion over a broadband of a wavelength band in use and distortion in signal waveforms due to the non-linear phenomenon are less, and which is suitable for wavelength multiplexed transmissions.
In order to achieve the above objects, the above problems are solved by the following constructions of the invention. That is, a first construction of the optical fiber of the invention is characterized in that, the optical fiber is formed so that the outer circumferential side of the center core is covered by a first side core, the outer circumferential side of said first side core is covered by a second side core, and the outer circumferential side of said second side core is covered by an outer cladding, wherein, when the maximum refractive index of the center core is n1, the minimum refractive index of the first side core is n2, the maximum refractive index of the second side core is n3, and the refractive index of the outer cladding is nc, n1 greater than n3 greater than nc greater than n2, and when the relative index difference of the center core from the outer cladding is xcex941, the relative index difference of the first side core from the outer cladding is xcex942, and the relative index difference of the second side core from the outer cladding is xcex943, 1.7%xe2x89xa6xcex941, xcex942xe2x89xa6xe2x88x920.3%, and 0.25%xe2x89xa6xcex943, and furthermore, a value A determined by dividing the diameter a1 of the center core by the diameter a2 of the first side core is 0.15xe2x89xa6Axe2x89xa60.5, and a value B determined by dividing the diameter a3 of the second side core by the diameter a2 of the first side core is 1 less than Bxe2x89xa62.
Preferably, the second side core has one or more extremely-large refractive index portions, and of the one or more extremely-large refractive index portions, maximum refractive index points are positioned at the side of the first side core from the center of the width in the direction of the diameter of the second side core.
More preferably, the outer circumferential side of the second side core is covered by an inner cladding, the outer circumferential side of said inner cladding is covered by the outer cladding, and the refractive index of the inner cladding is set to be smaller than that of the outer cladding.
In addition, chromatic dispersion in the wavelength band in use is set to be less than xe2x88x92120 ps/nm/km.
Furthermore, a D/S value determined by dividing the chromatic dispersion D in a wavelength band in use by the chromatic dispersion slope S is set to be 0 to 500 nm, more preferably, 0 to 300 nm.
It is extremely preferable that the wavelength band in use of the optical fiber of the invention is set to be a wavelength band of 1.55 xcexcm.
Moreover, a first construction of the optical transmission line of the invention is characterized in that the dispersion slope in the wavelength band in use is reduced to be almost zero by connecting the optical fiber of the invention to a single-mode optical fiber having zero dispersion in a wavelength band at the side of a wavelength shorter than the wavelength band in use.
Furthermore, a second construction of the optical transmission line of the invention is characterized in that the optical fiber of the abovementioned construction of the invention and an optical fiber whose D/S value determined by dividing chromatic dispersion D in the wavelength band in use by the chromatic dispersion slope S is larger than 300 nm or an optical fiber whose D/S value is negative are connected to a single-mode optical fiber having zero dispersion in a wavelength band at the side of a wavelength shorter than the wavelength band in use.
The present inventor noticed that the absolute value of negative dispersion per unit length in the wavelength band of 1.55 xcexcm could be made to be relatively large if the refractive index profile of an optical fiber was formed as a W-formed refractive index profile, and examined change in the absolute value of negative dispersion per unit length in the wavelength band of 1.55 xcexcm when the detailed structure of the W-shaped refractive index profile was changed. As a result, the inventor could confirm that the absolute value became larger when the core diameter including the center core and side core was reduced. However, in this case, it was proved that the light confinement effect into the center core with a high refractive index became weaker, and the light transmission conditions were not satisfied, so that light was not transmitted, or although light was transmitted, the macrobend loss increased extremely, and therefore, it became impossible to coil the optical fiber to form a dispersion compensated module.
Therefore, a method was established in that, at the outer circumference of the side core in the W-formed refractive index profile, a segment core having a refractive index higher than that of the side core was provided, and light which leaked from the center core of the W-formed refractive index profile and could not be transmitted was confined (not allowed to escape to the cladding side) by the segment core with a high refractive index, whereby the light was transmitted and the abovementioned macrobend loss was reduced. In addition, it was proved that, by this method, light was transmitted while spreading toward portions other than the center portion of the center core, whereby the effective core sectional area substantially became larger, and the power density of the light to be transmitted was reduced.
Based on the abovementioned examination, in the optical fiber of the invention, side core 12 in the W-formed refractive index profile is referred to as a first side core, the segment core is referred to as a second side core, and the relationship between the maximum refractive index n1 of the center core, the minimum refractive index n2 of the first side core, the maximum refractive index n3 of the second side core, and the refractive index nc of the outer cladding is set to n1 greater than n3 greater than nc greater than n2. In addition, the value A determined by dividing the diameter a1 of the center core by the diameter a2 of the first side core is set to 0.15xe2x89xa6Axe2x89xa60.5, the value B determined by dividing the diameter a3 of the second side core by the diameter a2 of the first side core is set to 1 less than Bxe2x89xa62, whereby enlargement of the effective core sectional area is made possible.
Furthermore, by concretely determining the relative index difference xcex941 of the center core from the outer cladding, the relative index difference xcex942 of the first side core from the outer cladding, and the relative index difference xcex943 of the second side core from the outer cladding to be 1.7% xe2x89xa6xcex941, xcex942 xe2x89xa6xe2x88x920.3%, and 0.25%xe2x89xa6xcex943, respectively, the dispersion value per unit length of the optical fiber can be made as small as, for example, less than xe2x88x92120 ps/nm/km (the absolute value of negative dispersion can be made larger), and the absolute value of the negative dispersion slope can be made larger.
By optimizing the refractive index profile based on the abovementioned examination by the present inventor, in the optical fiber of the invention, since the absolute value of negative dispersion per unit length in the wavelength band in use and the absolute value of the negative dispersion slope are made large, the optical fiber can be formed so that the positive dispersion of a 1.3 xcexcm band zero dispersion single-mode optical fiber can be compensated over a broadband of a wavelength band of 1.55 xcexcm, for example, or distortion due to the non-linear phenomenon can be reduced by enlarging the effective core sectional area.
In addition, according to the optical fiber arranged so that the second side core is provided with one or more extremely-large refractive index portions, in comparison with an optical fiber in which the second side core is flat in refractive index profile and has no extremely-large refractive index portions, the absolute value of negative dispersion and the absolute value of dispersion slope can be made large.
Furthermore, according to the optical fiber arranged so that the maximum refractive index points of one or more extremely-large refractive index portions of the second side core are at the side of the first side core from the center part in the diameter direction of the second side core, an optical fiber is more securely realized in which, while the effective cut-off wavelength is maintained at the short wavelength side, the absolute value of negative dispersion and the absolute value of the negative dispersion slope are made large.
Furthermore, according to an optical fiber arranged so that the outer circumferential side of the second side core is covered by an inner cladding, the outer circumferential side of said inner cladding is covered by an outer cladding, and the refractive index of the inner cladding is set to be smaller than that of the outer cladding, and by providing the inner cladding, light of an LP11 mode having electric field distribution over a wide range in the direction of the core diameter is made to easily leak to shorten the effective cut-off wavelength, whereby an optical fiber can be more securely realized in which the absolute value of negative dispersion and the absolute value of the negative dispersion slope are large while an operation in the single-mode can be performed without fail.
Furthermore, according to the optical fiber arranged so that chromatic dispersion in the wavelength band in use is set to be less than xe2x88x92120 ps/nm/km, the absolute value of the negative chromatic dispersion in the wavelength band in use is thus made large, whereby positive dispersion of a 1.3 xcexcm band zero dispersion single-mode optical fiber can be compensated by the short optical fiber.
Furthermore, according to the optical fiber, the D/S value of which is determined by dividing chromatic dispersion D in the wavelength band in use by the chromatic dispersion slope S is set to 0 to 500 nm, positive dispersion of an optical fiber such as a 1.3 xcexcm band zero dispersion single-mode optical fiber can be compensated by the short optical fiber. Particularly, according to an optical fiber whose D/S value is set to 0 to 300 nm, the positive dispersion and positive dispersion slope of a 1.3 xcexcm band zero dispersion single-mode optical fiber can be compensated by the optical fiber with a shorter length.
Furthermore, according to the optical fiber whose wavelength band in use is set to a wavelength band of 1.55 xcexcm, by applying this optical fiber for wavelength multiplexed optical transmissions using an optical amplifier equipped with an erbium doped optical fiber, the positive dispersion and positive dispersion slope of a 1.3 xcexcm band zero dispersion single-mode optical fiber can be compensated by the short optical fiber.
Furthermore, according to the optical transmission line of the invention, by using the abovementioned optical fiber, the dispersion and dispersion slope over a broadband of the wavelength band in use can be reduced to be almost zero, so that an optical transmission line suitable for wavelength multiplexed transmissions in which distortion due to dispersion is less can be obtained, and in particular, according to the optical transmission line arranged so that the dispersion slope and dispersion value in the wavelength band in use are reduced to be almost zero, by reducing both dispersion and dispersion slope to be almost zero, an optical transmission line extremely suitable for wavelength multiplexed transmissions in which distortion due to dispersion is almost zero.